Phase sensitive amplifier



June 14, 1960 K, F, CUFF PHASE SENSITIVE AMPLIFIER 2 Sheets-Sheet 1 Filed Sept. 14, 1959 TTT INVENTOR. KE/-M/T E CUI-'F Arr RNEYS June 14, 1960 Filed Sept. 14, 1959 K. F. CUFF PHASE: SENSITIVE AMPLIFIER 2 Sheets-Sheet 2 TUNABLE NARROW BANDPASS AMPLIFIER CALIBRATED AMPLITUDE VARIABLE STANDARD f- ATTENUATOR CIRCUIT Fig. 2

INVENTOR. KERM/T E CUFF United States Patent HASE SENSITIVE AMPLIFIER Kermit F. Cuit, Palo Alto, Calif., assigner, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Sept. 14, 1959, Ser. No. 839,984

6 Claims. (Cl. Z50-214) his invention relates to a thermo-electric receiver response tester, and more particularly to a thermo-electric receiver response tester utilizing the null type of indica tion and technique.

The need has long been felt for a simple reliable thermo-electric receiver response tester capable of testing a thermo-electric receiver over a wide range of interruption frequencies which is simple to operate and does not require an extensive amount of time in calibration setup or test runs.

According to the invention a thermo-electric receiver under test is placed in the radiation path of heat source such as a ribbon filament lamp. A photocell is placed in the radiation path of a light source such as an incandescent lamp. Both the heat and light sources are interrupted simultaneously by a chopper, which utilizes apertures varying sinusoidally with rotation of the chopper blade, While both the heat and light sources are interrupted sinusoidally by the same chopper, the variations are l80 out of phase from one another, i.e. the thermo-electric receiver is receiving maximum radiation from the heat source at the time the photocell is receiving minimum radiation from the light source. The output of the photocell is amplified, passed through a variable phase shifter, further amplitied, and passed through an amplitude standard circuit. This standard amplitude is then attenuated to a level approximating the output of the thermo-electric receiver, and passed through a narrow band-pass amplier which is tuned to the irequency of interruption of the chopper. The output of this ampliier is then passed through a second phase shifter, a phase splitter, and rectified in a synchronous rectier. The output of the rectifier is then ltered and metered. The synchronous rectier is synchronized by the output of the iirst variable phase shifter. The second phase shifter is adjusted to bring the signal to be rectied in phase with the synchronous rectifier. The output from the thermo-electric receiver is then compared with the output from the photocell channel after attenuation. The attenuator in the photocell channel can then be adjusted to balance out the output from the thermo-electric receiver as indicated by the null indicator. The nal phase adjustment can be made by the rst variable phase shifter to bring the output from the photocell and thermo-electric receiver precisely in opposite phase relationship. The variable attenuator in the photocell channel can be calibrated in voltage output to indicate the output or" the thermo-electric receiver. Obviously the speed of the rotation of the chopper can be varied necessitating merely a re-tuning of the bandpass ampliier and readjustment of the two phase Shifters to test the response of the thermo-electric receiver at diierent interruption frequencies.

1t is thus an object of the present invention to provide a thermo-electric receiver response tester utilizing a null type of indication.

Another object is the provision of a thermo-electric ice receiver response tester capable of use over a wide range of thermal interruption frequencies.

A further object of the invention is to provide a thermoelectric receiver response tester with a minimum of precision components.

Still another object is to provide a thermo-electric receiver response tester with self-contained and easily operated calibration.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same is better understood by reference to the following detailed description when considered in connection with accompanying drawings in which like reference numerals designate like parts throughout the iigures thereof and wherein:

Fig. 1 illustrates a block diagram of a preferred ernbodiment of the invention; and

Fig. 2 shows a schematic diagram of the major components of Fig. l.

Referring now to the drawings, there is shown in Fig. l a light source 11, a photocell 12 coupled to an amplier 13. The output from amplilier 13 is coupled to variable phase shifter 14 the output of which is coupled to current amplifier 16 and shaping circuit 17. The output of shaping circuit 17 is coupled to bistable multivibrator 18, the output of which is coupled to synchro-y nous rectiiier :19. The output of current ampliier 16 is fed through amplitude standard circuit 21, calibrated attenuator 22, and voltage divider 23 to summing circuit 24. Also coupled to summing circuit 24 is the output thermo-electric receiver 26 which is energized by heat source 27. The output of summing circuit 24 is coupled through tunable narrow band-pass amplier 28, variable phase shifter 29, and phase splitter 31 to synchronous rectier 19. The output of synchronous rectiiier 19 is taken through iilter 32 to null indicator 33.

Referring to Fig. 2, photo-electric cell 12 is connected between a positive voltage source and ground through load resistor 41. The junction of photo-electric cell 12 and resistor 41 is coupled through capacitor 42 to cathode follower 43. Plate 44 of cathode follower 43 is connected directly to a positive source. Grid 46 is connected through resistor 47 to the junction of resistors 43 and 49 connected in series between cathode 51 and ground. The junctions of resistors 48 and 49 is also connected through capacitor 52 to grid 53 of a resistance coupled amplifier indicated generally at 13. The output of amplier 13 is taken through load resistor 58 to B plus and also through variable resistor 59 and capacitor 61 to grid 62 of triode 63. Cathode 64 of triode 56 is connected through resistor 66 to ground and through capacitor 67 to grid 62. Grid 62 of triode 63 is also connected through resistor 68 to the junction of resistors 69 and 71. Resistors 69 and 71 are connected in series between cathode 72 and ground. Also connected to the junction of resistors 69 and 71 is capacitor 72, which is in turn connected through resistors 73 and 74 to ground. Contact 76 on resistor 73 is connected to grid 77 of triode 78. Resistor '79 is connected from cathode S11 of triode 78 to ground. Resistor S2 is connected between plate 83 of triode 78 and B plus. Also connected to plate 83 are resistors S4, the other end of which is connected to grid 86 of triode 87, and capacitor 8S which is connected through resistor S9 to grid 91 of triode 92. Resistor 93 is connected between grid 91 and ground. Also connected to grid 91 is plate 94 of diode 96 and cathode 97 of diode 98. Cathode 99 of diode 96 is connected to a positive voltage and plate 101 is connected directly to ground. Cathode 102 of triode 92 is connected through resistor 103 to ground. Plate 104 is connected through resistor 106 to B plus and through capacitor 107 to grid 510S of triode 109. Resistor 111 3 is connected between grid 108 and ground. Cathode 112 is connected through resistor 113 to ground and to cathode 114 of triode 116. Plate 117 is connected through resistor i118 to B plus, through resistor 119 to grid 121, Y and through variable resistor 122 and resi stor .123"i1r` nected directly to B plus. Grid 124is connected thronghj.

resistor 141 and resistorsA 142 andf14 3 to*af negative poflntal- .T heuntion of; .resistors 142 and .1.431 is' wie nected through variableV resisto1j 1r !-4 andlcapacitor 1'45 ffgdes 132-911@ 126, rsspectrely. @realm connected through resistors 148 and 1149 to null indicator 33j, rI he junctionof resistors 1 48 andj149 isY connected through cupacitor 7151 to ground; vTheV other side1of-rneterj33 is) connected to contact 1 52 on resistor 153.VV One endof'iesistor 153 is connectedthrough resistor(1S-intoy B plus andthe other end through resistor 156r` to ground.

Triodos 87, .88 and 89 are all connected'withV their respective cathodes, grids, and plates in parallel i.e. connected directly together. The plateslof triodes 87, 88V

and 89 areconnected throughresistor 157 to B plus and capacitor 158 to ground; The grids of triodes 8 7, 88 and 89 are connected through resistor 159 to t he junction of .resistors 161 andV 162 vwhich are connectedy in series between thec'athodes of triodes 87, 88 and 89 and gjr'ound. The junction resistors. 161 and 162 are alsoY cnnectedthrough capacitor 163l and resistor 164 to the input ofthe amplitudestandard circuit 21, The output oii'amplitude standard 21 isV coupled throughacalibrated variable attenuator 22 to a voltage Vdivider 2 3. Voltage divider 23 (Fig. 1') is shown'as resistorsV 23a and 23vbV which are connected in series between theoutput of` calibratedvariable attenuator 22 andground; The junction of resistors 23a and 23bj is .connectedto one side ofthermo-electric receiver 26. The other'side oftherrnoelectric receiver 26 is coupled through primary winding 166'of-transfor1ner 167 to ground. isolating resistor 168 is; connected between the grounded side of winding 166 andthe grounded' end of resistor 235.; Secondarywinding 169 isi' connectedbetween. groundandtheinput of tunablenarrow band-pass ampliiier 28; The' output o f rnuplier `28 is. connected through capacitorv 171 to'gn'd 172v ofrtriodef-173. Grid 172 isalsocorl-nectedY through resistor 174' to ground, and throughy resistors 1 76 andv 1 77 in series to' B' plus. The .junction ofresistors 176 and" 177 is` connected through capac itor 178 to ground 4 ground; Plate 209 is also connectedthrough4v capacitor 214 to grid 215 of triode 147. Grid 215 is also connected to resistor 2116 to ground. Cathode 217 of triode 147 is connected through resistor 218 to ground. Plate- 146 is connected through resistor 219 to B plus. Grid 215 is also connected to the junction of resistors 221 and 222, which are connectedin Vseriesbetween B plus and ground.

- Operation Referring again 1go-Eig. l, light source radiatesflight through aperture 7 of rotating chopper blade 8. Blade 8 is attached; to shaft 9, whichY is rotatingatja predetermined speed. The light passing.Y through aperture 7 impinges on photocell'12 which generateslan output proportional to the amount of light passing through aperture 7. Aperture 7 is a groove in disc 8, the width of whichvaries sinusoidally with rotation of disc 8. The outputj o f p hoio cel l 12 will then-be asine waveat a frequencydeterminedby the; speed of r ota t`i'cmV of disc 8.

This output is then amplified in ampliiier- 1- 3jandfpassed thwugh. variable Phase Shifter 14.- :The Output Y 0f variable phase shifter- 14 is then .shaped in to sync` pulses in 'Sliepi'na circuit. 17., .thdeteil Of Whfh -will be. .heranafter described, to trigger bistable multivibrator 1 8. HTheoutputsof bistable multivibrator-1 8 are coupled asA synchronizing signals to synchronous rectifier 19. The outp ut of variableph'ase shifter 1 4 is also amplified in curreni amplifier 16 and passed to amplitude; standard circuit 21 and calibrated atienuator 22: The amplitude standafd circuit 21 and calibrated attenuator 22 cane be any of the commercially available amplitude calibrator-s such as the-General Radio microvoiter. The amplitude standard circuit isy mefely an adjustable attehuator theoutput of which is metered at apart'icu-lar standard amplitude, for example onevolt; through a variable calibrated attenuato; indicated at L22. The output of Vvzuiable calibrated attenuator 2 2 is then reduced through voltage divider 23-and passed in to the input of summing circuit 24. At this point of time .shutter 11 is closed so that there-isn() outputV from thermo-electric receiver 26. The output ofsumming circuit 24' is then ampliiied in a conventionaltunableband; pass amplifier 28. Obviousiy, ampliter 28 is tuned to the interruption frequencyof chopper blade 8. The output of amplifier 28 is Ythen passed through a -variable phase shifter 29 tophase splitter 31. The output of phase splitter 31 .is then rectied in synchronous rectiiieif 19, l'tered in lter 32 and indicated at meterV 3 3. The initial calibration-at this point is; accomplished with variable phase shifter 29, i.e. variable'` phase shifter 29- is*V adjusted so that the output of phasesplitter 31.is exactly-synchronizeior in phase, with bistable multivibraa'ndjthroughresistors 179 and`181 to ground.:V The junctien'ofj resistors 179 and 181 is' connectedthrough ca'-V pacitor 182 toY cathode 183. Resistor 184Y isconnected` between cathode 183 and ground.,v Plate 1860i triode 1 73 is connected through resistors 187 and F188, in series, to B plus. connected through capacitor=189 to ground. Plate 186 isralso connected through variable'resistor 187 andV ca-j pajctor 188to grid 189 oftriode 191. Grid` 189 is also connected tothe junction of resistors 179' and 181. PlateA 192 connected through resistor 193and resistor 194, inf.` cries, to B p'lus. VThe Vjunction oi resistors 193 and 194'is'connect`ed through capacitor 196 to' ground. Plate.

19 2j.is connected through Variable resistor V197 andY calic'ltc'rlY 19810:)v grid 199 of triode 201. Grid 199 is als-coniiectedthrough capacitor 2.02` to cathode 203 otfid ,191', fid thrgh rsister 204 to ground. catholdefllisalsoconnecied through resistor 206 to'ground.

Cathode of tube 201 is connected tolresistorf208 to.

grgurgil.; Plate 209 Yis connectedthrough resistors 211 :1111212 .iin-series. toBplus.. 'Ihcjjunction of resistors 211 and 212 is connected through Vcapacitor 213 toi The junctionof resistors. 187 and 188 .is-

tor 18.A This overcomesanyphase shift present introduced through the Aamplifier 28. Y

' Once variable phase shifter 2 9is set which is done'by merely adjusting for a maximum indication a-null indicator 33, shutter 11is raised and heat source 27 isallowed to'radiate heat through V aperture 10 of chopper blade 8.

The heat passingV through aperture l1 0-will then impingeupon thermo-electric receiver 2 6, which can be' any of thewell knowntypes such a s athermo-couple or thermopile, .etc Thermo-electric receiver 26 will then generate an output voltage proportional tothe amount of heat passingthrough aperture 10. Aperture 10 is varied sinusoidally Yin width Withtherotation of choppernblade 8 but 180 outof phase the L aperture 7' ie.' Whenaper'- ture 1.0 is at a maximum width aperture 7 is atarninimun Thus, the out'putlrom themo-electric receiver '26 as seen by summing circuit" 24 .wi ll be 180 outfof'phase with the signal received with th otherr-input to circuit 241mm voltage divider 23'..y The calibrated v ariable' attnuator 22 can then' be adjusted for of thetwosignal inputs at summing tri-1cuit`24`.Y willy be indicated by a dip Eri-"null at null indicator 33. To

This output is passed compensate for any differences in phase or response time between photocell 12 and thermo-electric receiver 26 variable phase shifter 14 can also be adjusted at this point for a further null rendering the systems even more accurate.

Referring now to Fig, 2 the output from photo-electric cell 12 is developed across resistor 41 and applied through capacitor 42 to cathode follower 43. The output of cathode follower 43 taken through capacitor 49 is amplified through conventional amplier resistance coupled amplier 13 and applied to grid 55 of triode 56. Triodos 56, 63 and 78 comprise the variable phase shifter 14 of Fig. l. As will be understood, the phase shift is accomplished by varying the resistance of resistors 59 and the pick o point of resistor 73. Since resistors 59 and 73 form a part of an RC network, the phase will vary accordingly. The output of triode 73 is taken at plate 83 through resistor 89 to grid 91 of triode 92. Triodes 92 and diodes 96 and 98 comprise the shaping circuit indicated generally at 17 in Fig. l. Diode 9.8 is a conventional negative clipper and diode 96 is a biased positive clipper i.e. when the signal at the plate 94 rises above the potential applied to cathode 99, diode 96 will conduct, clamping grid 91 at the potential applied to cathode 99. Thus, the negative half of the wave form coupled through resistor 89 will be lost or clamped in diode 98, and the positive half will be limited or clipped by diode 96. This wave form will then be amplied in triode 92 which is a conventional class A cathode biased amplifier. The output taken in plate 104 through capacitor 107 will be differentiated, appearing in the grid 108 of triode 109 as a sharp going negative pulse. T riodes 109 and 116 together comprise a monostable multivibrator shown generally by block 18 in Fig. 1. Grid 121 is tied to a negative potential through resistor 120 holding triodes 116 at cut off. Triode 109 is then normally conducting having no xed bias applied. A negative pulse at grid 108 of triode 109 cuts triode 109 off raising the plate voltage at 117 and the voltage at grid 121 of triode 116, driving triode 116 into a conductive state. At this point the plate current of triode 116 through common cathode resistor 113 holds triode 109 at cut off. As capacitor 115 charges to the B plus potential through resistor 118 the current through resistor 120, which originally rendered tube 116 in a conducting state, will decrease to the point Where grid 121 Will again cut o tube 116, and tube 109, no longer having bias applied through common cathode resistor 113, will resume its original conducting state, being in a condition to receive the next negative pulse from plate 104. rIhe output of multivibrator 18 is taken from plates 111 and 127 to grids 131 and 124 of triodes 132 and 126 respectively. Triodes 126 and 132 comprise the synchronous rectifier indicated generally at 19 in Fig. l. Grids 124 and 131 are connected through resistors 143, 143, and resistors 133 and 135, respectively, to a negative potential. This xed bias holds tubes 126 and 132 below cut oi with no signal from multivibrator 18. Multivibrator 18 in etect gates tube 126 when its grid is positive, and tube 132 when grid 131 is positive. The output from triodes 126 and 132 is taken across common cathode load resistor 131 through a filter,

comprising resistors 148 and 149 and capacitor 151 to one side of meter 33. The other side of meter 33 is connected to a positive potential on a voltage divider consisting of resistors 153, 154 and 156, connected in series between ground and B plus. With no input, contact 152 on resistor 153 is adjusted until the voltage is equal on each side of meter 33. Meter 33 is preferably a zero center type of D.C. galvanometer. Signals originating from photo-electric cell 12 are attenuated to a level comparable to that generated in thermo-electric receiver 26. The output of voltage divider 23a and 23h i.e. their junction is then placed in series with voltage divider thermoelectric receiver 26 across primary 166 of transformer 167. At this point the two signals, i.e. from the calibrated variable attenuator 22 and from the thermo-electric receiver 26 will be 180 out of phase and in opposing relationship. The secondary winding 169 of transformer 167 is then coupled to the tunable narrow band-pass amplifier 28, which again is tuned to the frequency of chopper blade 8. The output of tunable narrow band-pass amplifier 28 is then taken through capacitor 171 to the input or control grid of triode 173. Triode 173 and triode 191 comprise the variable phase shifter indicated at 14 in Fig. 1. 'Ihe phase shift is again created by varying the resistance of resistors 187 and 197 which are in the output coupling circuits of triodes 173 and 191 respectively. Since these are RC coupling networks varying the resistance will vary the phase shift. The output of the phase shifter is then passed to the grid of RC resistance coupled amplifier 201 the output of which is taken from plate 209 through capacitor 214 to grid 215 of triode 147. Triode 147 is a phase splitter indicated generally as at 14 in Fig. l. The loans of phase splitter 147 are equal resistors 218 and 219, located in the cathode 217 and plate 146, respectively, of the tube. The outputs are coupled to the control grids 131 and 124 of synchronous rectiiier 19. Thus it is seen that only one-half of the signal will be coupled through the synchronous rectifier at a time, depending on which side of multivibrator 18 is conducting, and that maximum signal will appear at cathodes 138 and 139 of synchronous rectiiier 19 when the signals coupled from phase splitter 31 are exactly synchronized or in phase with the multivibrator 18 cycle.

Summary Again the initial calibration is made with shutter 11 isolating thermo-electric receiver 26 so that the only input seen at tunable narrow band-pass amplier 28 originates from photo-electric cell 12. Photo-electric cell 12 then generates sinusoidal signal of a frequency dependent upon the speed of rotation of chopper blade 8, this signal after being amplified 28 is fed through triodes 173 and 191 which comprise phase shifter 29 and into and through phase splitter 31 i.e. triode 147 and into synchronous -rectiiier 19 which is composed of triodes 126 and 132. At the same time the amplified signal from photo-electric cell 12 is taken it is clipped and clamped by diodes 96 and 98, respectively, and ampliiied through tube 92 to trigger multivibrator 18 which is composed of triodes 109 and 116. The outputs of multivibrator 18 gate synchronous `rectifier 19. Triodes 116 and 109 pass the signals from triode from phase splitter triode 147 to meter 33. With only an output from photo-electric cell 12 the critical phasing is that of the multivibrator as compared to the signals from phase splitter triode 147. This is accomplished by varying resistors 187 and 197 for a maximum indication on meter 33.

When shutter 11 is raised allowing heat source 27 to `radiate heat upon thermo-electric receiver 26 the voltage from thermo-electric receiver 26 is in phase opposition position to the output from calibrated variable attenuator 22. Calibrated variable attenuator 22 is then adjusted until the output of voltage divider 23 is equal in amplitude to the output from thermo-electric receiver 26. This will be indicated by a null reading of meter 33. Due to slight dierences in the response time of a thermoelectric receiver and a photoceil a slight phase difference may be present. Variable phase shifter 29, comprising triodes 56, 63, and 78, is adjusted to equalize this slight variation. This again will be indicated by a null in meter 33.

Thus a simple photo-electric receiver response tester has been disclosed which is inexpensive in construction, simple in operation, and variable over a wide range of interruption frequencies.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. For example, synchronous rectifier 19 could be ofthe relay or vibrator type, etc. It is therefore to be understood, that within the scope ofthe appended claims,

the invention may be practiced 'otherwise than as spepositioned for illuminating said'photocell, `a sine iunction chopper operable to interrupt said-heat source VVand said light source sinusoidally'in opposite phase relationship, a' lst amplifier connected to the output of said-photocell, alst variable phase shifting meansrconnected to .the output of said lst amplifier, calibratedlamplitude control means, connected to the output'of said lstfvariable phase shifting meanasumming means havinglst and 2nd inputs, said summing meanslst input connected to the output of ysaid thermo-electric" receiver, `said summing means 2nd input connected to the output of said amplitude` control meahs,ga' 2nd amplifier connected to the output of said 'summing means,A a 2nd variable phase shifting means connected to the output of said 2nd amplifier, a phase splitter connected to the output of said v2nd amplifier, rectifving means connected to the output lof said phase splitter, and a null indicator connected to the output of VVV2. A thermo-electric receiver -responsetester comprising a heat source in-thermal radiation transfer relationship With a thermo-electric receiver. Yunder test, a shutter operable to isolate said thermo-electric receiverfrom said heat source, alight source, a photocell, said-light sourcepositioned for illuminating said'photocell, a V sine-'function chopper operable to interrupt said heat source-and said 'light source sinusoidally in opposite phase relationship, a

lst'ampliiier connected to the output of-said photocell, a 1st variable phase shifting means connected to the output of said lst amplifier, calibratedamplitudefcontrol means connected to theoutput of said lst variable phase shifting means, a calibrated variable attenuator--connected tothe output of said amplitude control means, avoltage divider connected to the output ofV said variable attenuator, surnming means having Vlst Vand 2nd inputsfsaid summing means 1st input connected-'to the output of said thermoelectric receiver, said summing means-2nd input connected to the output of said voltage divider, a V2nd amplifier connected-to the output of-said summing means, a 2nd yvariable phase shifting-means-connected to the output of said Zndamplifien-a phase splitter-connected to the Voutput-o said 2nd amplifier, vrectifying means-connected Vto the output of `said phase splitter, anda null-indicator 'connectedto the output of said rectifying means. Y

'tionedl for 'illuminating said photocell, lafchopper, having av sine function chopper blade Yrotatablynrounted with the axis of rotationmidway betweensaid thermo-electric receiver and said photocell, said'blade interruptingjsaid tenseur@ er1`v saidlsllt sourwsnuscidallyin Opposite phase relationship, 'a istanaplifier connected to the output of said photecel l, a 1st ,variable phase shiftingmeans connected to the output of said lstam'plifi'e'r,1 calibrated amplitude control means, connected to the output Vof said 1st variable phase shifting means, summing means-having 1st and ZndinpuiS, Said summing means lst input connected to the output of Vsaid thermo-electric receiver,

said summing means Y2nd input connected to the output off-said amplitude control means, a 2nd amplifier connected to the, output of said summing means, a 2nd variablephase vshifting means connected Yto the output of said Zndam- ,plien' a phase splitter connected to the output of said VY2nd ampliiier, rectifying means connected to `the .output ofwsaid'phase splitter, and a null indicator connected to the output of--said iectifying means.

411i thermo-eletrjic receiverresponse tester. comprising a heat source in thermal radiation transferrelationship with a thermo-electric receiver under test, a shutter operable to isolate saidthermo-electric receiver `from isaid heat source, alight source, a photocell, said light source positioned for illuminating said photocell, a chopper hav- Yting ya sine function chopper blade rotatably mounted with Vthe airis-of rotation midway between said thermo-elecreceiverY and said photocell, said blade interrupting said'hcad sourceand said light source sinusoidally in opposite phase relationship,a lst amplier connectedto the output of said photocell, a lst variable phase Vshifting lmeans connected to the output Yof said ll-st amplifier, calibrated amplitude ycontrol means connected to the output ofsaid lst variablephaseshifting means, acalibrated variableattenuator connectedtov-the rnitputYY of said Aamplitude control means, a yvoltage divider connected :to fthe foutputofpsaid variable attenuator, summingmeans having 1 st and ,2nd, inputs, said summing means Alstinput connected tothe outputof said thermo-electric receiver,

`saidfsuniiniiig means '2nd-input connected to the output of said voltage divider, a 2nd amplifier connected Yto theoutput of s aidn summing means, a 2nd variable phase sln' ttir 1 g V means connectedto the output of said 2nd amplilier, a pl1 ase^ splitter connected to the output o fV said 2nd amplifier, rectifyinglmeans connected tor-the output of said phase splitter, land a null'indic-ator connected tothe output of said rectifying means.

5.v The thermo-electric receiver response tester; of claim Y4 Whereinsaidrectifying means comprises a synchronous rectifier andjmeans synchronizing said rectilierwith the output of said lst variable phase shifter.

v6. VThe thermo-electric receiver response tester of. claim 5 Wheiein. said I ast mentioned means comprises abistable multivibrator triggeredby the output of said -lst Hvariable pliasejshifter.

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